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Solar System

Causes of the seasons

Introduction: (Initial Observation)

The seasons are spring, summer, fall and winter.
Have you ever wondered why the weather temperature changes in different seasons? Why winters are cold and summers are hot? After all, the earth always gets the heat and light from the same sun.

If we get closer to a fire, we get hotter. Could it be then that the Earth gets closer to the Sun during Summer, and farther during Winter? Is it the angle of the earth and sun radiations that change? Is it the length of days that change?

In this project you will study the causes of seasons.

Come up with a testable hypothesis about the causes of seasons and then perform your experiments or observations.


This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.  

Project advisor

Project plan:

Before you start any project, you must write down your plan and describe the steps you are planning to take about your project. This is a sample:

  1. Gather information about the earth and the sun and possibly making a model for close observation.
  2. Gather information about what others think or suggest about the causes of the seasons.
  3. Propose a hypothesis (educated guess) about the causes of seasons.
  4. Designing and performing an experiment to test the hypothesis
  5. Reporting the results in data tables and graphs.

Information Gathering:

Find out about the earth and its revolutions around the sun. Also learn about the rotation axis of the earth. Read books, magazines or ask professionals who might know in order to learn about the cause of seasons. Find out what others suggest or think about the cause of seasons. Keep track of where you got your information from.

Following are samples of information you may find:

There is a popular misconception that the seasons on the Earth are caused by varying distances of the Earth from the Sun on its elliptical orbit. This is not correct. One way to see that this reasoning may be in error is to note that the seasons are out of phase in the Northern and Southern hemispheres: when it is Summer in the North it is Winter in the South.

Seasons in the Northern Hemisphere

The primary cause of the seasons is the 23.5 degree of the Earth’s rotation axis with respect to the plane of the ecliptic, as illustrated in the adjacent image (Source). This means that as the Earth goes around its orbit the Northern hemisphere is at various times oriented more toward and more away from the Sun, and likewise for the Southern hemisphere, as illustrated in the following figure.


Besides, that the Earth’s orbit is an ellipse, not a circle, so that at some times the Earth is closer to the Sun than at others; but this ellipse is very nearly a circle, and the relatively small differences in distance to the Sun cannot account for the changes in temperature. And to make things worse, the Earth is closer to the Sun during the northern hemisphere Winter!



The solstices are days when the Sun reaches its farthest northern and southern declinations. The winter solstice occurs on December 21 or 22 and marks the beginning of winter (this is the shortest day of the year). The summer solstice occurs on June 21 and marks the beginning of summer (this is the longest day of the year).


Equinoxes are days in which day and night are of equal duration. The two yearly equinoxes occur when the Sun crosses the celestial equator.

The vernal equinox occurs in late March (this is the beginning of spring in the Northern Hemisphere and the beginning of fall in the Southern Hemisphere); the autumnal equinox occurs in late September (this is the beginning of fall in the Northern Hemisphere and the beginning of spring in the Southern Hemisphere).


In the Northern Hemisphere, the winter solstice always occurs on or about December 21 and marks the beginning of the winter season. As many people notice, it’s the shortest day of the year, featuring the least amount of daylight between sunrise and sunset.

In the Southern Hemisphere, this is the time of the summer solstice and the longest day of the year. From now on, as the northern days grow longer so do the southern days get shorter.


Question/ Purpose:

What do you want to find out? Write a statement that describes what you want to do. Use your observations and questions to write the statement.

The purpose of this project is to find out about the causes of seasons. More specifically, we want to know if the angle of the sun rays changes and if this can be the cause of seasons.

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

This is a sample of how you may define variables:

  1. Independent variable is the day of the year.
  2. Dependent variable is the length of shadow. The length of shadow increases as the angle of sun rays decrease.
  3. Constants are the time of day. The height of the vertical bar we use to observe shadows.
  4. Controlled variable is the temperature. We record the temperature because large changes in the temperature may cause noticeable changes in the length of our vertical bar.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis.

The cause of seasons (hot summer and cold winter) must have something to do with the sun and the earth. After all sun is to only source of energy for the earth.

This is a sample hypothesis:

The angle of the sun rays has no effect on the weather temperature and there must be a different reason for the cause of seasons.

This is another hypothesis:

As the earth revolves around the sun, the angle of sunlight changes due to a tilt in the rotation axis of the earth. This change of sunlight angle is the cause of temperature change. If the angle of the sun is changing, then the size of shadows should change as well. Also when one hemisphere tilts away from the sun, the other hemisphere tilts towards the sun. as a result when there is winter in northern hemisphere, it should be summer in the southern hemisphere.

This is one more sample hypothesis:

In general, shadows are longer in winter and shorter in summer. After December 21st (Winter solstice) the shadows get shorter ever day. After June 21st (summer solstice) shadows get shorter everyday.

If we measure the length of mid day shadow casted on a flat surface in 7 days of January, then we will notice that each day the shadow is shorter than previous day.

Experiment Design:

Design an experiment to test each hypothesis. Make a step-by-step list of what you will do to answer each question. This list is called an experimental procedure. For an experiment to give answers you can trust, it must have a “control.” A control is an additional experimental trial or run. It is a separate experiment, done exactly like the others. The only difference is that no experimental variables are changed. A control is a neutral “reference point” for comparison that allows you to see what changing a variable does by comparing it to not changing anything. Dependable controls are sometimes very hard to develop. They can be the hardest part of a project. Without a control you cannot be sure that changing the variable causes your observations. A series of experiments that includes a control is called a “controlled experiment.”

Experiment 1: Does shadow length decrease when we are approaching summer?


In this experiment you will measure, record and compare shadow length of a vertical object in 7 different days. The vertical object may be a vertical bar that you install or a post that is already installed somewhere and you have access to that.


Mount a 4 feet tall (about 120 centimeters) wooden dowel in the ground in a sunny flat spot. Make sure that at mid day (12:00 noon) the end of the shadow is on a flat surface.

Every day for 7 days (or more) at mid day use a meter stick and measure the shadow length. Also mark it with a permanent marker.

Record the shadow length in a table like this:

Date Shadow length of a …..

Is the shadow length changing in the direction you predicted?

Note: It is easier to observe and measure the shadow length of taller objects.

Experiment 2: Does southern hemisphere have an opposite set of seasons in compare with the northern hemisphere?

Introduction: If the seasons are caused by a tilt in the earth rotation axis, then when it is winter in the northern hemisphere, it must be summer in the southern hemisphere. The reason is that when the northern hemisphere is tilted away from the sun, then the southern hemisphere is tilted toward the sun. For the observations and data collections of this project, you may use the Internet and the existing databases.


Use the internet resources and find the average temperature in different months for any major city in northern hemisphere (for example Chicago in USA) and write them in a table. Also use that to make a graph.

Do the same for a major country/city in the southern hemisphere (Buenos Aires/Argentina).

Your data table may look like this:

Month of the year Average temperature Chicago/ USA Average Temperature
Buenos Aires/ Argentina

Compare the temperatures in the above tables and make 2 overlapping line graphs to show the temperature changes in two places.

You may use any of the following websites for this study:



Your graph may look like this:

Additional Experiment 3:


Gathered information indicates that a 23.5º tilt in the earth rotational axis with respect to the plane of earth orbit is causing the difference in day length in different days of the year. As a result, when the Earth is at a certain place in its orbit, the northern hemisphere is tilted toward the Sun and experiences summer. Six months later, when the Earth is on the opposite side of the Sun, the northern hemisphere is tilted away from the Sun and experiences winter. The seasons are, of course, reversed for the southern hemisphere. So we design and make a model of earth with tilted axis and use it to observe the day length in different days or seasons of the year.


Get a piece of cardboard about 24″ x 24″ and install a light bulb in the center of that to represent the sun.

Draw a circle with the radius of about 10″ around the light bulb.

Divide the circle into 4 equal parts and insert four equidistance toothpicks in the circles perimeter in a way that all toothpicks are tilted to one direction about 23.5º.

Get some Styrofoam balls and paint them as earth models. Make sure to mark south and north poles and show some latitude lines. Try to draw the equator and 2 latitude lines (40º South and 40º North). You may paint them using a brush or insert them partially in a water based paint solution. Painting in this way requires a lot of hours waiting for each paint to dry before you can try the other one.



Place a Styrofoam ball over each toothpick such that the tooth pick passes the center of each ball and act as the earth rotation axis.

Turn on the light and mark the area of each latitude line that is illuminated by the light.

Light will represent the sun and each ball represents the earth in a different position of it’s orbit. Compare the illuminated section of each latitude line with dark section of the same latitude line for all four positions of earth around the sun.


Do this in a dark room. It will be easier to identify the areas that are dark. It will also be easier to see what portion of each latitude line in in the dark. Make a close observation and mark the seasons based on northern hemisphere.




Additional Experiment 4:


In this experiment you will use a globe as your earth model and use a flashlight or a desk lamp as the sun to see how does the tilt in the earth axis affect the length of the day. To change the direction of the tilt, simply rotate the base of the globe 180º. (You may also mount a vertical half toothpick on your globe and see how does the length of shadow change.)

When I got my globe, I noticed that it’s rotation axis is already tilted 23.5º and it is ready to use. Most likely yours will be tilted too.

Turn on the light on the desk and make it faced to the globe and at the same level or elevation. Turn the globe base in a way that the north pole be tilted toward the light.

Do you have more light on north hemisphere or south hemisphere?



Make a look from the top (or from north pole). See one of the latitude lines. What portion of latitude line is in the dark? Carefully mark the points in a latitude line where the light ends.

Use a meter stick or a ruler to measure the length of the specific latitude line which is in the light. Also measure the length of the dark portion of the same line. Use these to calculate the longest day length. The circle is drawn on latitude 55ºN.




Now turn the base of the globe in a way that the north pole be tilted away from the sun. Compare the light in northern hemisphere with the light in the southern hemisphere.

Compare the light in the north pole with the light in the south pole. What day of the year is this in the northern hemisphere? Is it winter or summer?





When the north pole is tilted away from the sun, look at the globe from the north pole direction. You can see the portion of the light in the northern hemisphere.

Make drawings or pictures from your experiment setup and experiment results and use them as a part of your display.

Is there any change in day light hours in equator in different seasons?

Materials and Equipment:

List of materials may be extracted from the experiment section.

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.


If you do any calculations, write your calculations in this part of your report.

Summery of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.
Your conclusion may include information like this:

When the northern hemisphere is leaning away from the sun, the rays coming from it hit this part of the Earth at a smaller angle than on other parts of the world. This means that the same amount of light is distributed over a larger surface, and therefore these places receive less heat than the others. At the same time the days get shorter when a hemisphere is tilted away from the sun. A combination of these two factors cause cold winters in the northern hemisphere.

On the other hand when the northern hemisphere is leaning toward the sun, the rays coming from it hit this part of the earth at a more direct angle (close to the right angle). This means that the same amount of light is distributed over a smaller surface and therefore these places receive more heat than the others. At the same time the days get longer when the northern hemisphere is tilted towards the sun. A combination of these two factors cause warm summers in the northern hemisphere.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


Visit your local library and find books related to the earth and the solar system. List the websites and the books you review as your references/ bibliography.

Seasons, Wikipedia Encyclopedia